Field of the Invention
The present invention relates to transmission line technology, and in particular relates to a maximum power output circuit for an EHC and a design method thereof.
Background Information
For real time monitoring of grid assets and effective reduction of grid faults, online grid monitoring systems are extensively developed at home and abroad. FIG. 1 shows the structure of such a system, where a monitoring device is directly installed on the power transmission line for monitoring of inclination angle, stress, conductor temperature and conductor current, the monitoring data thereof being transmitted wirelessly to a monitoring platform, which then accesses the status of the power transmission line with combined inputs of monitored parameters and running status of the transmission line. Practices in recent years show that power supply and communication are two bottlenecks impeding development of online monitoring solutions for power transmission lines.
Up to now, of mature harvesting solutions there are mostly solar energy, wind power, capacitive divider, laser supply, induction harvesting, differential temperature harvesting, and vibration harvesting. By comparison of the above-mentioned harvesting solutions, induction harvesting is believed to be the most suitable for transmission line energy harvesting. USI, OTLM, Hangzhou Thunderbird, and Xi'an Jinyuan have all developed commercial products based on induction harvesting. However, all the above products work on the range in excess of 50 A due to limited power supply, and hence are prevented from operating normally on most applications generally with a working current below 50 A.
For an online monitoring power source for power transmission line, it needs to be capable of adapting to big load swings in addition to posing no risk for the transmission line per se. Thus, an induction harvesting solution shall meet the following requirements for: {circle around (1)} large dynamic range; current over a power transmission line ranges from a peak current over 1000 A to a valley one of 40 A and even as low as 10 A for certain distribution networks; output power of an energy harvesting coil (hereunder abbreviated as EHC) is positively correlated with the current on the transmission line; as is shown on FIG. 2, the output power of the EHC needs to be regulated via practical means to consistently output a stable power within the wide dynamic range; {circle around (2)} high density per unit power; the weight of an online monitoring device is strictly regulated due to safety considerations, for example, the weight of a universal monitoring device is limited to 2.5 kg, that of a vibration monitoring is limited to 1 kg, and that for a distribution network monitoring device is limited to 500 g; therefore, the only way to solve the problem is to increase the power density of the energy harvester; and {circle around (3)} anti-surge capability; a transmission line is subject to impact of short circuits or lightning, which might result in a peak current of several kA, so the induction harvester shall be able to withstand such current surges.
Foreign and domestic scholars focus their research mostly on two aspects, power output model and protection of the EHC. N. M. Roscoe, M. D. Judd, L. Fraser, “A novel inductive electromagnetic energy harvester for condition monitoring sensors,” in Proc. Int. Conf. Condit. Monitor. Diagnosis, Sep. 2010, pp. 615-618, N. M. Roscoe, M. D. Judd, and J. Fitch, “Development of magnetic induction energy harvesting for condition monitoring,” in Proc. 44th Int. Univ. Power Eng. Conf., September 2009, pp. 1-5, N. M. Roscoe, Judd M. D. Harvesting energy from magnetic fields to power condition monitoring sensors.” IEEE Sensors J., vol. 13, no. 6, pp. 2263-2270, 2013, consider an EHC equivalent to a voltage source or a current source, with output power of the EHC reaching it maximum when load resistance is equal to internal resistance of the power source. In fact, output voltage of the EHC changes as the load current changes, and as the load of the EHC changes, its output voltage and current change simultaneously, and therefore the above assumption does not strictly hold.